Method for restoration of the drive torque during a change of gear

ABSTRACT

Method for restoration of the drive torque during a change of gear, wherein the clutch is closed in order to adjust the angular speed of the drive shaft to match the angular speed of the primary shaft of the gearbox; when the angular speed of the drive shaft is close to the angular speed of the primary shaft of the gearbox, a reference profile is generated for the angular speed of the drive shaft, and the drive torque generated by the engine is controlled in order to make the angular speed of the drive shaft follow the reference profile, which has a final portion which is substantially tangent to the angular speed of the primary shaft of the gearbox.

[0001] The present invention relates to a method for restoration of the drive torque during a change of gear, and in particular a change of gear carried out by a power-assisted control manual gearbox, to which the following description will refer specifically, without detracting from generality.

BACKGROUND OF THE INVENTION

[0002] Power-assisted control manual gearboxes are becoming increasingly common, which are structurally altogether similar to a manual gearbox of the conventional type, with the exception of the fact that pedals and control levers which can be actuated by the user are replaced by corresponding electric or hydraulic power-assisted controls. When using a power-assisted control manual gearbox, the user simply needs to transmit to a control system the order to change to a higher gear or a lower gear, and the control system automatically carries out the change of gear, by acting on the butterfly valve control and on the various power-assisted controls.

[0003] In normal driving conditions, a power-assisted control manual gearbox is required to guarantee a high level of comfort to the passengers of the vehicle during the phases of change of gear; recent studies have shown that in order to guarantee a high level of comfort for the passengers, the change of gear must take place within short times, and without giving rise to oscillations which can be perceived by the passengers themselves.

[0004] On completion of the operations of change of gear, the clutch must be reclosed, in order to render the drive shaft and the primary shaft of the gearbox integral with one another; in particular, after having established the new ratio in an ascending change (i.e. when the new ratio is “lengthier” than the preceding one), the vehicle proceeds at a speed which is virtually identical to the speed before the action of changing gear, the engine rotates at a speed which is substantially the same as that obtained for the vehicle by means of the preceding ratio, and the primary shaft of the gearbox rotates at a speed which is the same at that obtained for the vehicle by means of the new ratio, and thus rotates more slowly than the drive shaft.

[0005] In this situation, in order to reduce the change time to a minimum, the clutch should be closed in the shortest time possible; however, this operative method is acceptable only at the limit of sporting driving, since it induces a series of oscillations in the speed of travel of the vehicle, which can clearly be perceived by the passengers, with a consequent deterioration in running comfort.

[0006] In order to close the clutch on completion of the operations of change of gear, it is known to actuate progressively and simultaneously both the power-assisted control of the clutch, and the power-assisted control of the butterfly valve, such as to increase simultaneously and progressively both the torque transmitted by the clutch and the torque generated by the engine. However, it has been found that this methodology is capable of avoiding stresses which can be perceived by the passengers only if it is carried out within relatively very lengthy times.

SUMMARY OF THE INVENTION

[0007] The object of the present invention is to provide a method for restoration of the drive torque during a change of gear, which avoids the drawbacks described hereinabove and is, in particular, of easy and economical implementation.

[0008] According to the present invention, a method is provided for restoration of the drive torque during a change of gear, as described in claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention will now be described with reference to the attached drawings, which illustrate a non-limiting embodiment of it, in which:

[0010]FIG. 1 is a schematic view of the engine and of the drive units of a vehicle provided with a power-assisted manual gearbox;

[0011]FIG. 2 illustrates the temporal development of some quantities during an operation of reclosure of the clutch, carried out according to the method which is the subject of the present invention; and

[0012]FIG. 3 illustrates a block diagram of part of a control unit in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0013] In FIG. 1, 1 indicates as a whole the drive units of a known vehicle, which can transmit the motion from an engine 2 to a pair of drive wheels 3. The drive units 1 comprise a clutch 4, a power-assisted control manual gearbox 5, and a differential 6; in turn, the gearbox 5 comprises a primary shaft 7, which rotates at an angular speed _(ω1)(t), and a secondary shaft 8, which rotates at an angular speed _(ω2)(t), and transmits the motion to the drive wheels 3 by means of the differential 6 and a pair of axle shafts 9. By means of interposition of the clutch 3, the primary shaft 7 is connected to a drive shaft 10, which is rotated by the engine 2, and rotates at an angular speed _(ωm)(t).

[0014] The drive units 1 are also provided with a hydraulic power-assisted control 11 of a known type, which can control the clutch 4, and a hydraulic power-assisted control 12 of a known type, which can control the position of the secondary shaft 8, in order to determine the drive ratio which exists between the primary shaft 7 and the secondary shaft 8. The power-assisted controls 11 and 12 are controlled by a control unit 13, which is connected to a series of sensors (which are known and are not illustrated), in order to detect the commands of the driver, and to measure the value of some reference quantities of the engine 2 and of the drive units 1.

[0015] In a corresponding memory (which is known and is not illustrated) in the control unit 13, there is stored the function T_(cl)(x) of transmissivity of the clutch 4, which, for each position x of the clutch 4 (or rather for each position x of the disc thruster of the clutch 4) provides the torque T_(cl) transmitted by means of the clutch 4 itself, from the drive shaft 10 to the primary shaft 7 of the gearbox 5. In general, the function T_(cl)(x) of transmissivity of the clutch 4 can be obtained from the equation [0], in which F(x) is the force exerted by the disc thruster of the clutch 4, μ is the coefficient of friction between the discs of the clutch 4, and SIGN() is a binary sign function, and assumes the value ±1, depending on whether the angular speed _(ωm)(t) of the drive shaft 10 is greater or less than the angular speed _(ω1)(t) of the primary shaft 7.

T _(cl)(x)=F(x)*μ*SIGN(_(ωm)(t)−_(ω1)(t))  [0]

[0016] During a change of gear, from a current gear A to a subsequent, higher gear B (i.e. which has a lengthier transmission ratio), the control unit 13 commands the power-assisted controls 11 and 12, in order to carry out opening of the clutch 4, so as to release the gear A, to engage the gear B, and finally to close the clutch 4. During the above-described operations of change of gear, the control unit 13 keeps constantly under control the drive torque T_(m)(t) generated by the engine 2, in order to keep the angular speed _(ωm)(t) of the drive shaft 10 substantially equal to the required values.

[0017] During the phase of reclosure of the clutch 4, after the gear B has been engaged, the angular speed _(ωm)(t) of the drive shaft 10 is adjusted to match the angular speed _(ω1)(t) of the primary shaft 7, which angular speed _(ω1)(t) is set by the speed of the vehicle, since the primary shaft 7 is angularly integral with the drive wheels 3, by means of the axle shafts 9, the differential 6, the secondary shaft 8, and the gear train of the gear B. During the phase of reclosure of the clutch 4, the clutch 4 itself slides, and transmits torque T_(cl)(t) between the drive shaft 10 and the primary shaft 7; in this situation, the law of motion is provided by the equation [1], in which J_(m) represents the moment of inertia of the engine 2, and _(ω′m)(t) represents the drift over a period of time of the angular speed _(ωm)(t) of the drive shaft 10, i.e. the angular acceleration of the drive shaft 10.

J _(m)*_(ω′m)(t)=T _(m)(t)−T _(cl)(t)  [1]

[0018] Two other equations are derived directly from the equation [1]:

_(ω′m)(t)=(T _(m)(t)−T _(cl)(t)/J _(m)  [2]

T _(cl)(t)=T _(m)(t)−J _(m)* _(ω′m)(t)  [3]

[0019] In order to carry out relatively fast reclosure of the clutch 4, at the same time guaranteeing the comfort of the passengers, it is advantageous to regulate the engine 2 in order to prevent generation of useful torque (in fact, in these conditions, the engine generates drive torque T_(m)(t), which is slightly negative owing to the effect of the friction torques), and recloses the clutch 4 only partially, such as to make the clutch 4 itself transmit constant, predetermined torque T_(cl)* to the primary shaft 7 of the gearbox, for an interval of time which is sufficient to use up the excess kinetic energy of the drive shaft 10, and to make the drive shaft 10 rotate substantially at an angular speed _(ωm)(t) which is equal to the angular speed _(ω1)(t) of the primary shaft 7. In fact, in these conditions, the engine 2 generates mechanical energy which is lower than that required for the traction, and is transmitted by the clutch 4, and the energy deficit is recuperated by discharging the kinetic energy possessed by the drive shaft 10, which slows down.

[0020] When the drive shaft 10 reaches an angular speed _(ωm)(t) which is close to the angular speed _(ω1)(t) of the primary shaft 7, the engine 2 is regulated in order progressively to restore generation of positive drive torque T_(m)(t); when the angular speed _(ωm)(t) of the drive shaft 10 is very close to, i.e. substantially the same as, the angular speed _(ω1)(t) of the primary shaft 7, the clutch 4 is fully closed, and the change of gear is consequently completed.

[0021] In other words, the method for closure of the clutch 4 consists of adjusting the clutch 4 rapidly to a predetermined position x*, in order to transmit constant torque T_(cl)* (which is substantially equal to the drive torque T* supplied by the engine 2 immediately before the change of gear), and to keep the clutch 4 in the predetermined position x* until synchronisation takes place between the drive shaft 10 and the primary shaft 7. In particular, the engine 2 is regulated in order to supply zero torque T_(m)(t) (or, more generally, torque which is lower than the torque T_(cl)* transmitted by the clutch 4), until the angular speed _(ωm)(t) of the drive shaft 10 is close to the angular speed _(ω1)(t) of the primary shaft 7; at this point, the engine 2 is regulated in order to increase progressively the torque T_(m)(t) supplied, and the clutch 4 is reclosed fully, only when the angular speed _(ωm)(t) of the drive shaft is substantially equal (i.e. very close) to the angular speed _(ω1)(t) of the primary shaft (7).

[0022] In general, before the change of gear, the vehicle has acceleration a* which is produced by a drive torque T* supplied by the engine 2, by the effect of the driving actions of the driver; in addition, in order to guarantee maximum comfort, it is necessary for the operations of change of gear to cause the minimum disturbance to the travel of the vehicle. It is thus advantageous for the torque T_(cl)* transmitted by the clutch 4 during the phase of slowing down of the drive shaft 10 to be substantially equal to the torque T*, in order to keep the law of motion of the vehicle unaltered, and to prevent causing disturbance to the passengers.

[0023] During the operations of change of gear, the control unit 13 determines the value of the drive torque T* supplied by the engine 2 before the change of gear, and via the function T_(cl)(x) of transmissivity of the clutch 4, determines the position x* to which the clutch 4 itself (or rather the disc thruster of the clutch 4) must be adjusted, in order to transmit torque T_(cl)* which is substantially equal to the torque T* (minus the friction torques).

[0024] It is important to note that from the point of view of the drive wheels 3 (i.e. from the point of view of the vehicle) the operations of change of gear are concluded at the instant at which the clutch 4 has been adjusted to the position x*, in order to transmit the torque T_(cl)*, since from that instant onwards the drive wheels receive once again the torque T* which they were receiving before the change of gear. This torque T* is obviously supplied by the engine 2, and in an initial phase it is produced to the detriment of the kinetic energy of the drive shaft 10 (which consequently slows down in order to synchronise with the primary shaft 7), whereas in a subsequent phase, it is produced once again by the engine 2, by means of combustion of the fuel.

[0025] It is apparent from the foregoing description that traction is restored to the vehicle before the change of gear is completed definitively, with full reclosure of the clutch 4, since the drive wheels 3 receive full traction as soon as the phase of synchronisation of the drive shaft 10 with the primary shaft 7 begins, since, already during this phase, the primary shaft 7 receives from the clutch 4 torque T_(cl)* which is substantially equal to the torque T* received before the change of gear. By this means, from the point of view of the vehicle, the actual duration of the change of gear is reduced, since the phase of zero traction torque is reduced.

[0026]FIG. 2 illustrates the temporal development of the position x(t) of the clutch 4, and the temporal development of the angular speed _(ωm)(t) of the drive shaft 10, during the phase of reclosure of the clutch 4; in particular, engagement of the gear B is completed at an instant t₀, corresponding to which the clutch 4 is adjusted rapidly from a position of opening to a predetermined intermediate position x*, in order to transmit constant torque T_(cl)* (substantially equal to the drive torque T* supplied by the engine 2, immediately before the change of gear). The clutch 4 is kept in the predetermined position x* until an instant t₂, in which synchronisation of the drive shaft 10 and the primary shaft 7 is completed; at the instant t₂, the clutch is reclosed fully, and the change of gear is completed.

[0027] Between the instant to, at which the clutch 4 is adjusted rapidly from a position of full opening to the predetermined intermediate position x*, in order to transmit the constant torque T_(cl)*, and the instant t₁, at which the angular speed _(ωm)(t) of the drive shaft 10 is close to the angular speed _(ω1)(t) of the primary shaft 7 (i e. between the two speeds _(ωm)(t) and _(ω1)(t), there is a difference of between 50 and 200 rpm), and the engine 2 is regulated by the control unit 13, in order to supply zero torque T_(m)(t) (or, more generally, torque which is lower than the torque T_(cl)* transmitted by the clutch 4). Starting from the instant t₁, at which the angular speed _(ωm)(t) of the drive shaft 10 is close to the angular speed _(ω1)(t) of the primary shaft 7, the engine 2 is regulated by the control unit 13, in order to increase progressively the torque T_(m)(t) supplied, such as to vary the angular speed _(ωm)(t) of the drive shaft 10 according to a law of motion of a parabolic type, which is substantially tangent to the angular speed _(ω1)(t) of the primary shaft 7 (angular speed _(ω1)(t) substantially constant in the interval of time considered).

[0028] From the foregoing information, it is apparent that the angular speed _(ωm)(t) of the drive shaft 10 before the instant t₀ has a drift _(ω′m)(t) over a period of time which is constant and slightly negative, as can be seen clearly from the equation [2], since the torque T_(cl)(t) transmitted by the clutch 4 is zero, and the drive torque T_(m)(t) generated by the engine 2 is slightly negative, owing to the effect of the friction torques (the power supply to the engine 2 is stopped, and the engine is not producing useful torque). Thus, the angular speed _(ωm)(t) of the drive shaft 10 before the instant t₀ has a linear law of motion, with a slightly negative gradient.

[0029] The angular speed _(ωm)(t) of the drive shaft 10 between the instant t₀ and the instant t₁ has a drift _(ω′m)(t) over a period of time which is constant and is strongly negative, as can be seen clearly from the equation [2], since the torque T_(cl)(t) transmitted by the clutch 4 is constant, and is equal to the drive torque T* supplied by the engine 2, immediately before the change of gear, and the drive torque T_(m)(t) generated by the engine 2 is slightly negative, owing to the effect of the friction torques (the power supply to the engine 2 is stopped, and the engine is not producing useful torque). Thus, the angular speed _(ωm)(t) of the drive shaft 10 between the instant t₀ and the instant t₁ has a linear law of motion, with a pronounced negative gradient.

[0030] The angular speed _(ωm)(t) of the drive shaft 10 between the instant t₁ and the instant t₂ has a negative drift _(ω′m)(t), and a modulus which decreases in a linear manner over a period of time, owing to the effect of the progressive increase of the drive torque T_(m)(t) generated by the engine 2, under the control of the control unit 13. As a result of the equation [2], the torque T_(cl)(t) transmitted by the clutch 4 is constant, and is equal to the drive torque T* supplied by the engine 2, immediately before the change of gear, and the drive torque T_(m)(t) generated by the engine 2 increases in a linear manner, owing to the effect of the regulations applied by the control unit 13. Thus, the angular speed _(ωm)(t) of the drive shaft 10 between the instant t₁ and the instant t₂ has a parabolic law of motion; in fact, if the drift _(ω′m)(t) of the angular speed _(ωm)(t) has a linear increase over a period of time, the angular speed _(ωm)(t) has a law of motion of a parabolic type.

[0031] According to a different embodiment, the law of motion of the angular speed _(ωm)(t) need not be a law of a parabolic type. However, use of a law of a parabolic type is particularly advantageous, since the law of a parabolic type makes it possible to adjust the angular speed _(ωm)(t) until it is synchronised with the angular speed _(ω1)(t) in an extremely gentle and gradual manner (since the parabola of the angular speed _(ωm)(t) is designated such as to be substantially tangent to the straight line of the angular speed _(ω1)(t); in addition, the creation of a parabolic law of motion is relatively simple, since it requires the drift _(ω′m)(t) of the angular speed _(ωm)(t) to have an increase which is linear over a period of time, i.e. (on the basis of the equation [2]), it requires the drive torque T_(m)(t) generated by the engine 2 to have an increase which is linear over a period of time.

[0032] As illustrated in FIG. 3, the control unit 13 comprises a generation block 14, which, starting from the instant t₁, generates a reference signal _(ωmrif)(t), which indicates the ideal development which the angular speed _(ωm)(t) should assume between the instant t₁ and the instant t₂. The control unit 13 additionally comprises a piloting block 15, which receives as input the reference signal _(ωmrif)(t), the reading of the angular speed _(ωm)(t), the reading of the drift _(ω′m)(t) of the angular speed _(ωm)(t), and the reading of the torque T_(cl)(t) transmitted by the clutch 4, and can pilot the engine 2 such as to make the angular speed _(ωm)(t) follow the development of the reference signal _(ωmrif)(t), within a predetermined tolerance.

[0033] The piloting block 15 controls the torque of the engine 2, i.e. it communicates to a control system (which is known and not illustrated) of the engine 2 the value of the objective torque T_(mob)(t) which the engine 2 must supply, such that the angular speed _(ωm)(t) follows the development of the reference signal _(ωmrif)(t) . The value of the objective torque T_(mob)(t) is determined by the piloting block 15, by means of the sum of two contributions T_(mob1)(t) and T_(mob2)(t) which are independent from one another; the value T_(mob1)(t) is determined by means of an open-loop control logic, on the basis of the torque T_(cl)(t) transmitted by the clutch 4, whereas the value T_(mob2)(t) is determined by means of a closed-loop logic, on the basis of the difference between the angular speed _(ωm)(t) and the reference signal _(ωmrif)(t) In greater detail, the value of the objective torque T_(mob)(t) is determined by applying the equations [4], [5] and [6] listed below, in which K is the coefficient of gain, which depends on the operative condition.

T _(mob)(t)=T _(mob1)(t)+T _(mob2)(t)  [4]

T _(mob1)(t)=K*(_(ωmrif)(t)−_(ωm)(t))  [5]

T _(mob2)(t)=T _(cl)(t)+J _(m)*_(ω′m)(t)  [6]

[0034] By varying the value of the coefficient K of gain, and/or of the development of the reference signal _(ωmrif)(t), it is possible to vary both the duration of the interval of time which elapses between the instant t₁ and the instant t₂ (i.e. of the phase of resupply of torque on completion of the change of gear), and the gradient with which the angular speed _(ωm)(t) of the drive shaft 10 approaches the angular speed _(ω1)(t) of the primary shaft 7. It is important to note that the actual duration of the phase of resupply of torque substantially does not have any effect on the dynamic behaviour of the vehicle, since the traction on the drive wheels 3 is already restored starting from the instant t₁, as previously described. 

1) Method for restoration of the drive torque during a change of gear, wherein the clutch (4) is closed in order to adjust the angular speed (_(ωm)) of the drive shaft (10) to match the angular speed (_(ω1)) of the primary shaft (7) of the gearbox (5); the method consists of generating a reference profile (_(ωmrif)) for the angular speed (_(ωm)) of the drive shaft (10), and controlling the engine (2), in order to make the angular speed (_(ωm)) of the drive shaft (10) follow the reference profile (_(ωmrif)). 2) Method according to claim 1, wherein the said reference profile (_(ωmrif)) is generated when the angular speed (_(ωm)) of the drive shaft (10) is close to the angular speed (_(ωm1)) of the primary shaft (7) of the gearbox (5). 3) Method according to claim 1, wherein the said reference profile (_(ωmrif)) has a final portion which is substantially tangent to the angular speed (_(ω1)) of the primary shaft (7) of the gearbox (5). 4) Method according to claim 3, wherein the said reference profile (_(ωmrif)) has a final portion which is of a parabolic type, and is substantially tangent to the angular speed (_(ω1)) of the primary shaft (7) of the gearbox (5). 5) Method according to claim 1, wherein the drive torque (T_(m)) generated by the engine (2) is controlled, in order to make the angular speed (_(ωm)) of the drive shaft (10) follow the reference profile (_(ωmrif)). 6) Method according to claim 5, wherein, in order to control the drive torque (T_(m)) generated by the engine (2), a torque reference value (T_(mob)) is generated, to be requested from the engine (2) itself. 7) Method according to claim 6, wherein the said torque reference value (T_(mob)) comprises a first open-loop contribution (T_(mob1)), which is dependent on the torque (T_(cl)) transmitted by the said clutch (4). 8) Method according to claim 7, wherein the said first contribution (T_(mob1)) is calculated as the sum of the torque (T_(cl)) transmitted by the said clutch (4), and the product of the moment of inertia (J_(m)) of the engine (2) and the angular acceleration (_(107 ′m)) of the drive shaft (10). 9) Method according to claim 6, wherein the said torque reference value ( T_(mob)) comprises a second contribution (T_(mob2)) in a closed loop, which is dependent on the difference between the angular speed (_(ωm)) of the drive shaft (10) and the reference profile (_(ωmrif)). 10) Method according to claim 1, wherein the clutch (4) is adjusted to a predetermined position (x*), in order to transmit constant torque (T_(cl)*), and the clutch itself is kept in the predetermined position (x*) until synchronisation has taken place between the drive shaft (10) and the primary shaft (7). 11) Method according to claim 1, wherein, in the said predetermined position (x*), the clutch (4) transmits constant torque (T_(cl)*) which is substantially equal to the drive torque (T*) supplied by the engine (2) immediately before the change of gear. 12) Method according to claim 10, wherein the power supply to the said engine (2) is stopped, so as not to generate useful torque until the angular speed (_(ωm)) of the drive shaft (10) is close to the angular speed (_(ω1)) of the primary shaft (7) of the gearbox, and the said engine is regulated in order to make the angular speed (_(ωm)) of the drive shaft (10) follow the reference profile (_(ωmrif)), when the angular speed (_(ωm)) of the drive shaft (10) is close to the angular speed (_(ω1)) of the primary shaft (7) of the gearbox (5). 13) Method according to claim 10, wherein the said clutch (4) is reclosed fully only when the angular speed (_(ωm)) of the drive shaft (10) is substantially equal to the angular speed (_(ω1)) of the primary shaft (7). 